The present invention relates to a process for producing a semiconductor device, more particularly, to a process including the step of introducing boron impurities into a semiconductor body in self alignment, utilizing a silicon nitride film.
In a process for producing a semiconductor device, a silicon nitride film is first selectively removed to form windows conforming to the dimensions of predetermined elements of the semiconductor device, and subsequently, impurities including boron are introduced through the windows, and onto the semiconductor body surface to form the device elements in self alignment with the selectively removed silicon nitride film. Various semiconductor device structures, for example an iso-planar structure produced by self alignment, have been invented for the purpose of reducing the size of a semiconductor element to be as small as possible and, hence, increasing the integration density of integrated circuits. The defining borders of a film including the silicon nitride film, are hereinafter referred to as patterning. In the production of a bipolar transistor by utilizing self alignment, a silicon nitride film is patterned so as to form windows exposing a silicon dioxide film beneath the silicon nitride film. Then windows exposing the isolation region, the base region and the collector contact region of a silicon layer are successively formed through the silicon dioxide film, using the silicon nitride film as a mask, and then boron impurities are diffused through each window. All of the windows through the silicon nitride film, exposing the silicon dioxide film, are simultaneously formed to prevent deviation from the desired positions of the three regions mentioned above.
One step for producing a bipolar transistor utilizing self alignment is illustrated in FIG. 1, wherein an isolation region is formed in the NPN bipolar transistor utilizing the conventional self alignment method. In FIG. 1, an N type epitaxial layer (2) is formed on a P type semiconductor substrate (1), and a silicon nitride film (3) is patterned as illustrated in FIG. 1. A silicon dioxide film (4) is formed on the portions of the N type epitaxial layer (2) not covered by the silicon nitride film (3) and is then removed from the isolation region (5). The P type isolation region (5) is formed by gas diffusion using boron bromide (BBr.sub.3) as a diffusion source.
It was discovered in experiments conducted by the present inventors that when boron, a commonly used P type impurity, is diffused, the boron reacts with the silicon nitride (Si.sub.3 N.sub.4) film (3) and generates a disadvantageous phenomenon of the adhering of the reaction mixture on the silicon dioxide film and, thus, degrading the silicon dioxide (SiO.sub.2) film. The silicon dioxide film (4) masking one region (6a) is degraded during the successive boron diffusion into the isolation region (5) and the region (6b) of the N type epitaxial layer (2), with the result that boron is caused to diffuse into this region (6a) of the N type epitaxial layer (2) and thus may form a P type region. In extreme cases, the resistance of a region, where a high resistance is to be provided, is abnormally decreased due to this disadvantageous phenomenon.
In recent years, the dimensions of semiconductor elements, such as bipolar transistors, have decrease and, the integration density of semiconductor devices has increased. Thus the surface area of a semiconductor substrate occupied by semiconductor elements is smaller than the area occupied by the metal conductors which connnect the semiconductor elements to each other. Since the metal conductors are formed on the insulating film of the silicon nitride covering the semiconductor substrate, the surface area of the silicon nitride film is larger than the surface area of the semiconductor elements and increases the integration density. Consequently, the degrading of the silicon dioxide film phenomenon due to the reaction between the boron and the silicon nitride film leads to a reduction in the production yield and the reliability of the semiconductor devices.
In order to avoid degrading the silicon dioxide film, measures for protecting the silicon nitride film against reacting with boron have been employed by the applicant. In these methods the silicon nitride film is covered by a silicon dioxide film which is deposited on the silicon nitride film by a CVD process. However, in these measures the CVD step is necessary, in addition to the conventional steps of producing the semiconductor devices, and thus the production cost of the semiconductor devices is high.